With the engine under normal loads of the truck, any time I let off the gas I hear this odd harmonic burble/gurgle from the engine. I have heard it with the truck sitting still and me revving it from under the hood but not nearly enough to be able to pick it up on a video. I noticed it more once I put on my K&N filter so I am lead to believe that this is something intake related. Possibly the PCV valve? IDK...

Any input or experience would be appreciated.

(Oh, its a 2002 4.6L)

 

Oh, and just before I noted this noise it was/is throwing a "Bank 1 too lean" code. I think it was Bank 1... I cant remember. I dismissed is since it was running/driving ok.

 

Bueller.... Bueller...

If this little tidbit helps- I replaced the large vacuum line from the valve cover to the back of the I/M due to collapse. I heard this noise before this and was hoping the code thrown and the noise was this hose. After replacement, the noise actually got a little more noticable.